This invention relates generally to monitoring operation of an engine and, more particularly, to monitoring operation of an engine and generating maintenance messages for both out-of-range and in-range fault conditions.
Engine control units (ECUs) detect sensor system problems and generate maintenance messages using hardware and software optimized for capturing hard faults, i.e. open or short circuit conditions that remain unchanged for an extended period of time. Typically a hard fault results from a mechanical problem in a sensor, a cable, or the ECU data acquisition hardware. The ECU has validation logic which sets a validation flag to an INVALID state if a signal is "out-of-range".
The ECU also has sensor selection logic which monitors the validation flags of input signals and selects a best signal for use by the engine control logic. For example, if a validation flag for one sensor is set to INVALID and dual sensor inputs are provided to the ECU, the ECU sensor selection logic checks the validity of the other sensor. If the validation flag of the other sensor is set to VALID, then the sensor selection logic selects the signal from this "in range" sensor. If both sensors fail, i.e., both flags are set to INVALID, or if a second sensor is not available, then the ECU sensor selection logic selects a model or fail-safe value. If the validation flags for both sensors are set to VALID, then the selected value is typically the average of the two sensor values.
The sensor selection logic also generates a selection status (SST) number which indicates a type of fault accommodation that has been taken. The SST number is then used by maintenance logic to generate maintenance messages. Specifically, if a fault condition SST number is set for a specific period of time, generally referred to as a persistence time, then a maintenance message is transmitted to an aircraft central maintenance system. Post-flight maintenance actions typically are based on these messages.
The above described ECU does not generate maintenance messages for fault conditions due to intermittent mechanical problems or for many "in-range" fault conditions. Maintenance messages are not generated for intermittent conditions since under such conditions, the signal does not remain in the "out-of-range" state for a time period greater than the persistence time requirement (typically 10 seconds). Maintenance messages also are not generated for many faults that cause the signal to shift but remain "in-range", i.e., in-range faults. Therefore, for such faults, the validation logic flag will not be set to an INVALID state and the sensor selection logic will select the shifted signal unless a difference test can be made using another sensor. Even if a second sensor is available and the sensor selection logic sensor-to-sensor difference limit is exceeded, a maintenance message may still not be generated since the shifted signal must remain shifted for a time period greater than the persistence time requirement. This type of signal variability is possible since "in-range" faults are typically caused by a time-varying resistance in series with the input signal or between the signal line and ground.
By not having the ability to generate maintenance messages for these fault conditions, i.e., faults due to intermittent conditions and "in-range" faults, the ability to trend a fault condition and predict when a hard fault will occur is not available. This prediction capability is particularly useful for maintenance planning since many engine fault conditions are capable of causing aircraft delays and cancellations.